1. Field of the Invention
The invention relates to an exhaust gas control apparatus for an internal combustion engine.
2. Description of the Related Art
When an internal combustion engine is started, the engine temperature and the intake air temperature are low and the fuel is less likely to evaporate. Accordingly, a control for making the air-fuel ratio richer than the stoichiometric air-fuel ratio is executed. Therefore, an oxygen-deficient condition develops and a large amount of hydrocarbon (HC), which is an unburned fuel component, is discharged from the internal combustion engine. For a brief period after the internal combustion engine is started, an exhaust gas catalyst does not remove such a large amount of HC because the temperature of the exhaust gas catalyst is low. Therefore, technologies for addressing this problem have been developed. According to these technologies, an adsorption member that temporarily adsorbs HC is provided in an exhaust passage, and HC in the exhaust gas is trapped on the adsorption member until the temperature of the exhaust gas catalyst is increased to a sufficiently high temperature.
As the exhaust gas is introduced into the adsorption member, the temperature of the adsorption member gradually increases. Then, when the temperature of the adsorption member becomes equal to or higher than a predetermined desorption temperature, desorption of the HC, which has been adsorbed on the adsorption member, starts. Accordingly, in order to prevent the HC from being released into the atmosphere, the exhaust gas catalyst needs to be ready to remove the HC, that is, the temperature of the exhaust gas catalyst needs to be increased to the activation temperature, before the temperature of the adsorption member reaches the desorption temperature.
According to an existing technology, secondary air used to oxidize (burn) HC in the exhaust gas is supplied to an exhaust system in order to quickly increase the temperature of a catalyst after an internal combustion engine is started. If the secondary air is supplied to the exhaust system, the HC is burned in the exhaust system (afterburning of the HC occurs) and the exhaust gas temperature increases. As a result, the catalyst is warmed quickly.
Japanese Patent Application Publication No. 10-169434 (JP-A-10-169434) (refer to paragraph [0017]) describes an exhaust gas control apparatus which includes an adsorption member and a catalyst provided upstream of the adsorption member, and in which the secondary air is introduced to an exhaust system at a portion upstream of the catalyst during a period from when an internal combustion engine is started until when the catalyst is activated. However, the technology described in JP-A-10-169434 has the following problem. If the capacity of the adsorption member is fixed, as the flow rate of exhaust gas that passes through the adsorption member increases, the HC adsorption rate decreases. Therefore, if the flow rate of exhaust gas that passes through the adsorption member increases due to introduction of the secondary air into the exhaust system, the HC adsorption rate decreases. As described above, for a brief period after the internal combustion engine is started, a large amount of HC is discharged from the internal combustion engine. In this case, if the HC adsorption rate has been decreased due to supply of the secondary air, the amount of HC that is released into the atmosphere without being adsorbed on the adsorption member increases.
If supply of the secondary air is started when an engine is started, afterburning of HC is less likely to occur because the temperature of exhaust ports is still low. Japanese Patent Application Publication No. 2004-124824 (JP-A-2004-124824) describes a technology for addressing this problem. According to JP-A-2004-124824, during start-up of an engine, supply of the secondary air is prohibited until the temperature of exhaust ports becomes equal to or higher than a predetermined temperature so that the exhaust ports are prevented from being cooled by the secondary air.
JP-A-2004-124824 describes that if supply of the secondary air is started when the temperature of the exhaust ports becomes equal to or higher than the predetermined temperature, afterburning of HC reliably occurs and the exhaust ports are prevented from being cooled by the secondary air.
However, when the HC adsorption state is taken into account, if the time when supply of the secondary air is started is determined based on the temperature of the exhaust ports, the determined time is not always the optimum one. That is, if the amount of HC that is discharged from the engine reaches its peak after the temperature of the exhaust ports increases to the predetermined temperature and supply of the secondary air is started, a large amount of HC flows into the adsorption member in the state where the HC adsorption rate has been decreased due to an increase in the flow rate of the exhaust gas. Therefore, the HC is more likely to be released into the atmosphere without being adsorbed on the adsorption member.
According to the technology described in JP-A-2004-124824, if the increase in the temperature of the exhaust ports is delayed, the time when supply of the secondary air is started is delayed by an amount corresponding to a delay of the increase in the temperature. In this case, the time when promotion of catalyst warm-up due to supply of the secondary air becomes effective is delayed, and therefore the time when the catalyst is activated is also delayed. This makes it difficult to increase the temperature of the catalyst to a temperature equal to or higher than the activation temperature before the temperature of the adsorption member exceeds the desorption temperature. As a result, the amount of HC that is released into the atmosphere is likely to increase.